Natural gas in recent years has been playing an increasingly important role as an energy source for industrial and building-heating applications, in the generation of electric power and in many other cases. The natural gas as recovered from the ground comprises, in addition to methane, several percent by volume of higher hydrocarbons.
It is frequently desirable to recover these higher hydrocarbons either for use in the synthesis of still other organic compounds or for collection of a C.sub.3 /C.sub.4 fraction which can be recovered as a liquid and can be used as liquefied gas for various industrial and commercial applications, e.g. as the fuel for cigarette lighters, household appliances, torches or the like.
In addition, the natural gas contains significant quantities of sulfur compounds, usually in the form of hydrogen sulfide (H.sub.2 S) and carbonyl sulfide (COS), as well as carbon dioxide (CO.sub.2) in an amount up to 20%. Present in the natural gas may also be organosulphur compounds such as mercaptans, which can also be considered higher hydrocarbons because they generally have a carbon count of three or more.
It is generally considered advantageous, usually essential, to remove the H.sub.2 S, COS and CO.sub.2 from the natural gas to leave a methane-rich component which is delivered for use in the industry, power plant, chemical and heating operations mentioned previously.
While there are many reasons for this requirement, it can be readily appreciated that carbon dioxide, if present in the methane-rich gas, acts as a diluent which contributes no useful energy and merely increases the volume of the gas which must be processed or handled. Its presence, therefore, calls for an increase in the size of the apparatus, e.g. boiler, burners and pipelines, for a given generation of heat and thus increases the cost of the plant utilizing the natural gas. In fact, it functions as an agent reducing the heat value of the natural gas.
Furthermore, removal of the CO.sub.2 can contribute advantages on other levels, since the carbon dioxide in relatively pure compressed or solid form, is itself a valuable commodity which can be used for many purposes, e.g. the production of dry ice, as a participant in reactions in which carbon dioxide is necessary, etc.
It is equally obvious that hydrogen sulfide must be removed from the natural gas before it is used as a fuel to avoid high sulfide dioxide levels in the combustion of exhaust gases with the concomitant detriment to the environment and the combustion apparatus. On the other side hydrogen sulfide is a valuable product which can be used for the production of elemental sulfur. The same holds true for the carbonoxysulfide or carbonyl sulfide (COS).
It is known to remove hydrogen sulfide and carbon dioxide from natural gas by subjecting the latter in successive stages to scrubbing operations.
The scrubbing solution used in the first stage is a liquid which is partially charged with CO.sub.2 and which preferentially scrubs H.sub.2 S from the natural gas. This liquid is then subjected to complete regeneration and is used to scrub CO.sub.2 out of the gas in the second stage, thereby being partly charged with CO.sub.2 and adapted to be used in the first stage.
This system has the disadvantage that both the said gas components, namely H.sub.2 S and CO.sub.2 are collected in a single scrubbing solution, thereby complicating the regenerating process if the two are to be recovered independently.
The regeneration cost is significant and with conventional processes it is not generally possible to recover the two acid gas components individually with high purity, nor is a recovery of higher carbons possible.